This invention relates to a novel and improved blending method and apparatus adaptable for use in fracturing oil and gas well formations, and more particularly relates to a high capacity blending apparatus which is capable of achieving a proper blend of liquid-to-liquid or liquid-to-solid constituents in a single stage operation.
Oil and gas wells are fractured customarily by introduction of acids and gel compositions in multiple steps or a series of operations. At least certain of those steps require the introduction of solid granular or particulate material which must be thoroughly intermixed with a liquid prior to pumping into the formation. For instance, in the hydraulic fracturing of certain sandstones, typically a blender draws water from a series of storage tanks to intermix with sand, polymers or other chemical additives. The mixture is pumped under pressure deep into the subsurface formation through a perforated well casing to fracture the surrounding rock. When the polymerized liquid is later withdrawn from the formation, the sand is left in place to prop open the fracture. Gas or oil may then flow through the fracture to the well bore and into the pipe line for distribution.
In the past, among other approaches taken in blending of liquid and solid materials, generally the solids and liquids are intermixed by a paddle in a large open tub as a preliminary to pumping into the formation; or the liquids and solids are mixed together before they are advanced through the impeller zone of a blender. Moreover, conventional blending apparatus has generally required multi-stage blending, particularly in order to mix rather large quantities of liquid and solids or additives and to maintain them in suspension when pumped over the extended distances necessary to fracture the subsurface formations of the earth. Representative of such blenders which have been utilized in multi-stage operations is the patent to Zingg et al U.S. Pat. No. 3,256,181 wherein the liquid and particulate material are intermixed by swirling the liquid to impart a rotary motion and introducing the solid or particulate material near the center of rotation of the liquid and discharging the materials through an impeller under sufficient velocity to the solids to intimately mix with the swirling liquid. Here the impeller is employed both to cause swirling of the liquid in developing a predetermined head of pressure while introducing the solid material either into the same impeller or to a second rotating impeller for intermixture with the liquids at the second stage of the impeller unit. Blending apparatus has been employed also in mixing of cement and drilling mud in single stage operations and generally representative of such techniques are the U.S. Letters Patent to Owsley U.S. Pat. No. 2,147,053 and Raglan U.S. Pat. No. 2,626,788. In neither case however is the impeller so arranged that the solid inlet of the impeller is isolated from the liquid stream as the liquid stream is separately introduced in an axial direction toward the discharge end of the apparatus as solids are discharged by the centrifugal force of the impeller vanes into the liquid stream.